You have too many fish for 7500 gallons and nothing like enough filteration by the description given. You need to get rid of about 70-80 fish first off otherwise this dangerous level of over-crowding will cause lots of extra stress which your fish don't need. 1 fish per 250 gallons is the absolute minimum requirement in my view but 500 or 1000 gallons per fish is better.
You need to turn over between 3750 and 7500 gallons an hour through a mechanical and biological filter assembly. Typical bought assemblies consist of a vortex and biological chambers containing media on which good bacteria grow. The chambers are either in series via pipework or contained in a large round container such as the nexus range for example.
I warn you now Guy, decent filters cost money, several hundred pounds upwards but they are easy to install compared to building your own filters and you need a quick remedy, based upon what you say. Installing a decent filter will help to keep the water quality up, remove debris and waste inorganic compounds such as ammonia (produced by the fish gills and in urine) nitrite (converted from ammonia by good bacteria) and nitrate (converted from nitrite by good bacteria) which helps feed algae.
Join a local Koi club and ask local Koi keepers for advice, particularly if your fish are showing signs of ill-health.
Posted on Apr 6, 2005, 6:22 PM from IP address 213.120.101.21